1. Field of the Invention
This invention relates to an optical scanning apparatus and an image forming apparatus using the same, and particularly is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine or a multi-function printer having, for example, an electrophotographic process adapted to deflect a beam emitted from light source means by a polygon mirror as a light deflector (deflecting means), and optically scan a surface to be scanned through the intermediary of an imaging optical system having fθ characteristic and a plurality of reflecting mirrors to thereby record image information.
2. Related Background Art
In recent years, in an imaging optical system (scanning optical system) and an image forming apparatus including the same, a higher speed and compactness have been advanced, and in order to further enhance an additional value, a further improvement in performance and structure for further compactness have been studied.
Also, regarding the resolution of an image, an image of higher definition such as 300 dpi, 600 dpi, and further 1,200 dpi is required.
To make an image so high in definition, it is necessary in designing an imaging optical system to realize higher performance by making a scanning magnification and a spot diameter at each image height being constant, and reducing the curvature of scanning lines and the curvature of image field, and further accuracy is also required in working accuracy such as the eccentricity of each lens and the disposition accuracy of an optical box.
Particularly in a color image forming apparatus of a type in which a plurality of color lights are superimposed one upon another on a plurality of photosensitive drums and a color image is output, the amount of curvature of scanning lines is important in effecting color register, and if for example, the amount of curvature of scanning lines differs on the plurality of photosensitive drums, there will arise the problem that when superimposition is effected, color misregistration occurs and the quality of image is lowered.
Also, in achieving compactness, there has heretofore been adopted a technique of making the imaging optical system wider in angle of view and shorter in focal length to thereby shorten an optical path, or bending an optical path disposed in a straight line by a mirror (reflecting mirror) to thereby make an occupied area smaller.
It is the surface accuracy of the mirror that particularly needs care when the optical path is bent by the mirror to thereby achieve compactness. If the surface accuracy is not sufficient (less planarity), there will arise the problem that the optical performance as an imaging optical system is deteriorated.
What are particularly problematic are a change in imaging magnification at each image height due to the curvature of the surface of the mirror, and the curvature of scanning lines. If, for example, the reflecting surface of the mirror which should be of a planar shape forms a convex surface, the magnification becomes an enlargement magnification, and in the end portion of the image in a main scanning direction, the scanning magnification is enlarged and the spot interval in the main scanning direction is not constant. Also in a sub-scanning direction, if the mirror is likewise of a convex surface shape, the optical path length and the angle of reflection change at the center and at the end portions of the image and the imaging position deviates to cause the curvature of scanning lines.
Therefore, the surface accuracy of the mirror need be severely controlled, and when configured by a plurality of mirrors, further high accuracy becomes necessary, and this has been a factor of a cost increasing.
Heretofore, for example, as a technique of suppressing the curvature of scanning lines, there has been disclosed a technique of positively giving the amount of curvature of one of a plurality of mirrors by adjustment to thereby correct the curvature of scanning lines (see, for example, Japanese Patent Application Laid-Open No. 2001-228427).
Basically, however, even if one mirror is adjusted, a certain accuracy is still required for the other mirrors, and this leads to the problem that the work required for the adjustment thereof depends on the accuracy of the other constructed mirrors, therefore, the amount of adjustment does not become constant and structure also becomes complicated.